16 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



season. The stalks are erect in some species, spreading in others, and 

 a few inches to 8 feet or more long, but they are always straight and with 

 few or no branches. The bast fibers are long and stronger than in most 

 native plants, for which reason they have been examined to some extent 

 with a view to their utilization in the manufacture of cordage and cloth. 



The leaves are nearly always opposite or whorled and occur at 

 regular intervals to the top, the upper leaves being nearly as large as 

 the lower ones. It is not unusual, however, for the lowest and the 

 topmost leaves to be alternate on the stem, and in a few cases all of 

 the leaves are greatly reduced in size. In A. subulata this reduction 

 is carried to so great an extent that for the greater part of the year the 

 plant consists only of naked, rush-like stems. Many of the species 

 are clothed with a woolly tomentum of plant-hairs, while others are 

 quite smooth and naked. 



The flowers appear in clusters, technically called umbels, in the 

 axUs of the upper leaves and often at the apex of the stem. Their 

 structure is exceedingly complicated as the result of adaptation to 

 insect pollination. The principal pollinators are bees, wasps, ants, 

 and flies of various species. It is quite certain that any species of 

 milkweed would fail to set seed if grown where the particular insect 

 necessary for its poUination is absent. The flowers, at least of some 

 species, produce an abundance of nectar, and bee-keepers report an 

 average production of 50 pounds of honey of good quality per colony 

 of bees year after year from some species, especially A. syriaca. 



The flowers are followed by pods, and each pod is fiUed with numer- 

 ous seeds. The seeds bear tufts of soft, downy hairs at the apex, 

 this down being especially noticeable as the pod opens, when the hairs 

 may be from 1 to nearly 2 inches long. Natiu-al propagation is chiefly 

 by seeding. Germination tests of two collections of seed of Asclepias 

 mexicana showed that 60 per cent were viable; one collection of A. 

 vertidllata gave 64 per cent, one of A. sullivanti gave 16 per cent, and 

 in one collection of A. syriaca no germination was effected. The seeds 

 are slow to germinate, and some difficulty has been experienced in 

 getting field-sown seeds to grow at all. More favorable results are to be 

 expected from seed gathered at time of full maturity. Alternating tem- 

 peratures seem to be necessary for successful germination in some cases. 



Several of the species, especially those of the narrow-leaved group, 

 are poisonous to stock, as has been demonstrated by Hall and Yates 

 (1915) and by Marsh et al. (1920). On the other hand, the young 

 shoots and young pods, at least of A. syriaca, are cooked and eaten 

 with impunity. 



The genus is characterized by the presence of a milky sap or latex, 

 which is carried in special vessels of the laticiferous tissue. As in the 

 case of Hevea, Ficus, etc., the rubber is borne in the latex, but the 

 plants are so small that extraction by a process of tapping, such as is 



